In Frequency Modulation (FM), a sinusoidal carrier signal of constant amplitude and frequency is modulated by an input signal of a lower frequency and of varying amplitude. FM thereby produces an output signal that is constant in amplitude, varying in frequency in accordance with the input signal, and within a specified frequency range called the deviation bandwidth. In particular, the instantaneous amplitude of the input signal is linearly transformed into a change d.omega. in the instantaneous frequency .omega.(t) of the carrier frequency w.sub.c. To recover the input modulating signal from the output modulation signal, frequency demodulation must be performed using an FM demodulator.
FM demodulators are well known, and consist of devices such as ratio detectors, Foster Seeley discriminators, phase-locked loop detectors, pulse-counting detectors, and quadrature or coincidence detectors. All of these demodulators--whether implemented as analog or digital apparatus--pass data to post-processing stages, and ultimately to an output amplifier.
For example, the Heathkit AJ-1510 Digital FM Tuner employs a digital discrimination technique for demodulating a frequency modulated signal. The discriminator is of the pulse position modulation type, is inductorless and diodeless, and contains two integrated circuits: a retriggerable monostable multivibrator, and an operational amplifier. An input signal at the retriggerable monostable multivibrator causes it to change states for a fixed period of time, as determined by an RC network to provide a sequence of pulses of constant width and amplitude that are generated at about one-half of the IF rate. Each pulse represents a zero-crossing event. Signal information is represented as deviations in the frequency of the zero-crossing pulses from a constant IF frequency.
In a pulse integration type of FM demodulator, the frequency modulated signals typically ar amplified and "hard-limited" to produce square waves which have zero-crossings spaced in the same manner as the zero-crossings of the FM signals. The square waves are then converted into a sequence of constant width and amplitude pulses, one pulse for each zero-crossing of the modulated input signal. Each pulse is integrated (or filtered) and subsequently differentiated to reproduce the modulating input signal information.
There are pulse integration demodulators that employ a single one-shot multivibrator that is triggered at each zero-crossing. However, recovery time difficulties are encountered during high frequency operation because the internal delay of the multivibrator approaches the period of the high frequency signals as the operating frequency is increased.
In another form of pulse integration demodulator, a source of frequency modulated signals is coupled to a coincidence detector by a first and second signal path. The first and second signal paths have unequal signal delay characteristics, so that the coincidence detector provides an output signal that includes a series of constant width pulses, wherein pulse width is determined by a difference in signal delay between the first and second signal paths. A low pass filter is coupled to the coincidence detector to recover the signal modulation represented by the series of constant width pulses. However, this form of pulse integrator exhibits operating disadvantages due to non-linearity of the integrating network which impairs its ability to perform sufficiently precise integration on the applied signal pulse train.